This invention relates to a mechanism for selectively transferring between predetermined paths pursuant to the insertion of a measurement or monitoring instrument into the core of a nuclear reactor.
The monitoring of a nuclear reactor in a power station or the like to determine such parameters as neutron flux density and distribution, temperature, etc. is necessarily performed remotely to minimize radiation exposure risks, and is commonly implemented by the insertion of a flexible elongate cable, similar to a medical catheter, into the core of the reactor. The cable mounts or embodies an appropriate measuring instrument/device at its remote end.
To enable a plurality of different core sites to be sequentially accessed, a branching out network of cable insertion paths is typically provided, with a path transfer mechanism being disposed at each branch point similar to a railway track switch.
A conventional such transfer mechanism is shown in FIGS. 1 and 2, wherein the bulbous, swivel end portion of an inlet guide pipe or tube 1 for a monitoring cable extends through an access slot in a rectangular housing 2 and is disposed within the flared end of a guideway 9 defined in and extending through a selector block 3 slidably disposed in the housing. A pair of spaced outlet guide tubes 5a, 5b communicate with equally spaced apertures extending through the opposite or lower sidewall of the housing via coupler fittings 6. The distal ends of the outlet tubes extend either directly or via further branch paths to measurement sites within the reactor core (not shown). An electromagnetic actuator 4, such as a solenoid, selectively drives the block 3 between the limit positions shown in FIGS. 1 and 2 via a core rod 7 to thus establish communication or a continuous insertion path between the inlet tube 1 and one of the outlet tubes 5a, 5b. An adjustable stopper 8 is provided to establish the limit position of the selector block 3 shown in FIG. 2, wherein the cable insertion/withdrawal path leads through the outlet tube 5b.
Such a conventional transfer mechanism construction requires very precise and thus costly machining due to the close tolerances that must be provided between the slide surfaces of the selector block 3 and corresponding surfaces of the housing 2. Moreover, even with such closely machined tolerances, contaminant powders and abrasive particles frictionally dislodged during the numerous insertions and withdrawals of the monitoring cable become increasingly trapped in the interface between the block 3 and the housing 2, which leads to excessive wear, malfunctioning, and transfer position inaccuracies. Finally, with such a conventional construction the inlet tube 1 must necessarily be bent at some point along its length during each transfer operation in order to assume the different angular orientations shown in FIGS. 1 and 2, which leads to premature fatigue ruptures and requires frequent maintenance inspections.